1. Technical Field
The present invention relates to tracking the location of containers that are carried on ships, railroad cars or trucks, or stored in freight yards. More particularly, the present invention relates to tracking and inventory of containers using a satellite Global Positioning System (GPS) and an Inertial Navigation System (INS). The INS can be replaced or supported by a combination of inertial sensors, speed sensors, and sensors indicating rotation or movement direction in combination with the GPS.
2. Related Art
Position or location tracking is a crucial component of many inventory and resource monitoring and management systems. Typical location tracking systems employ real-time positioning sensors that continuously or periodically provide position solutions for location tracking. These sensors or systems commonly acquire locations of vehicles, equipment, or inventory based on principles of either triangulation or proximity with respect to known locations using various electronic positioning means such as a Global Positioning System (GPS), Differential Global Positioning System (DGPS), Integrated Differential Global Positioning System and Inertial Navigation System (DGPS/INS), Real Time Locating System (RTLS), RTLS/GPS, RTLS/INS, transponders, ultra wideband locating system, or some combinations of the above systems.
For example, U.S. Pat. No. 6,577,921 discloses a container tracking system that tracks the real-time positions of the container handling equipment using GPS, INS and wireless communication. U.S. Pat. No. 6,657,586 describes a Real Time Locating system and method for locating an object with a tag attached to the object and with remote readers each with a GPS receiver. U.S. Pat. No. 6,266,008 discloses a system and method for determining the location of freight containers in a freight yard by attaching GPS receivers to each container. U.S. Pat. No. 6,611,755 describes a timing control method for a fleet management system using a communication, status sensor and positioning system. U.S. Pat. No. 6,876,326 discloses a location tracking system using communication search mode ranging techniques.
Limitations in physics, however, generally prevent the real-time positioning systems from achieving 100% reliability or accuracy. Examples of those limitations with respect to radio-wave positioning are: obstacles blocking line of sight position signals, or signals reflected from near-by surfaces (multi-path). Further practical limitations in sensor technologies include biases in measurements, or poor signal to noise ratio resulting from environment sources. These limitations result in common positioning errors such as inaccuracies, loss of position, or location drifts causing erroneous position data.
To overcome the physical and practical limitations, many real-time positioning systems employ complimentary sensors, or digital maps to improve accuracy and reliability. As an example, the complimentary nature of Inertial Navigation System (INS) and Global Positioning System (GPS) are the main reasons why the integrated GPS/INS system is becoming increasingly popular. The high, long-term accuracy of GPS can be combined with the high output rate, robustness and reliability of INS to deliver superior positioning performance. Depending on how information is shared and processed between GPS, INS and the integration computer, the integrated system architecture can be classified into three categories: loosely coupled system, tightly coupled system and deeply coupled (ultra-tight coupled) system. All these integration methods improve the real-time positioning performance.
In addition to INS systems to complement GPS, other components have been used for navigation of vehicles or aircraft to provide better measurements or estimations of the current positions. For example, U.S. Pat. Nos. 6,731,237, 6,697,736, 6,694,260, 6,516,272, 6,427,122, and 6,317,688 describe various techniques to integrate GPS systems with inertial sensors or units (gyros and accelerometers), altimeters, compass, or magnetometers using various linear and nonlinear filters to improve either reliability or accuracy of real-time positioning. U.S. Pat. Nos. 6,826,478, 6,801,159, and 6,615,136 disclose various techniques to increase the real-time INS positioning accuracy or correct the real-time error by incorporating stored map and location information, second sensor data, or predetermined perimeter threshold. U.S. Pat. No. 6,810,324 increases the real-time positioning accuracy by substituting high quality position measurements with upgraded low quality position measurement when the high quality measurement is not available. U.S. Pat. No. 6,853,687 describes a method to improve the real-time performance of the RTLS by incorporating magnetic field proximity-based pingers into the RFID tags. U.S. Pat. Nos. 6,766,247, 6,728,637, and 6,615,135 disclose various specific methods to increase positioning accuracy by incorporating map or route information in a GPS or other sensor.
But these solutions do not solve one of the important problems in the inventory and resource tracking environment: What happens when the real-time position solution is inaccurate, missing or is lost. And what happens after such erroneous information is reported or entered into an inventory database. As a simple example, a real-time positioning system based on an expansive tightly integrated GPS/INS solution can drift away from the true positions when the system enters an area with less than four GPS satellite coverage for a long period of time. In a typical inventory or resource tracking environment, inaccurate location measurements, if not corrected in time, can generate and propagate into widespread inventory or database errors. This occurs especially when tracking the position of containers or vehicles in a warehouse, container yard, or rail yard where tracking signals can be blocked. Resultant errors often require manual correction. The corrupted inventory database thus creates delays and often expensive corrective measures in resource management and inventory controls.
To correct for errors encountered even when GPS is combined with another system, such as INS, post-processed positioning techniques have been used to apply geographic information to obtain accurate survey position solutions. For example, U.S. Pat. No. 6,804,621 describes post-processed methods for aligning measured track data with locations on a digital map to correct geographic map locations.
Post-processing of position information can identify embedded unknown parameters and noises, and resolve the past position solutions. It would be desirable to provide a system that monitors real-time position data of an object such as a cargo container, and performs automatic post-processing to correct position data when signals are blocked or distorted in a timely fashion to provide position data with a high confidence level.